Where did you find this information regarding what (Coffeelake) CPU can do 4000Mhz?

What makes you think coffeelake can't run ram at 4000MHz ?

He's using the i7-8700k which the i7's (usually) have a stronger IMC than the i3's - i5's.
The Maximus X Hero supports up to 4133MHz.

G.Skill is producing up to 4600MHz kits for coffeelake and in this article they have an i5 (which I imagine is the 8600k) running 32GB at 4000MHz with the Maximus X Hero, this should settle any doubts you may have.

On the CPUs I have tested, the IMCs were variable. As a result, I do not expect that all will manage DDR4-4000 unconditionally stable on a four-slot board. Even on a two-slot board, I had one doing over 4300, while the other barely managed 4133.

On the Hero X I tried to run this kit on two different 8700k's https://www.gskill.com/en/product/f4-4000c18d-16gtzr at the XMP speeds and neither would boot. Post code error 55 'Memory not installed'. Same at 3866, but booted at 3733 and 3600. Have adjusted timings at 3600 to match the C16-3600 G.Skill kit and has passed memtest86 overnight. Tried tweaking VCCIO, VSA, 'Maximus Tweak' mode, and DRAM voltage, with no success at 4000.
Unsurprisingly for this kit no Z370 boards are listed under QVL on the G.Skill site.

Intel should come up with a way to make their cpu's more consistent so the i7's can run at least 16GB of what the board is rated. As it stands you're more or less taking a chance with (the more expensive) high speed ram. I know it's based on the silicon lottery but we the people should feel more secure about purchasing high speed ram. Sure we can bin cpu's but that gets expensive fast and in the end you can only use one cpu.

So what would Intel need, new more accurate machines that makes the cpu's ? It's not like they're starving for money and I feel it could be done.

Intel rates coffee lake for up to 64GB at 2666MHz, that covers their butt pretty good. But when the motherboard supports up to 4133MHz the cpu shouldn't be the limiting factor.

Intel should come up with a way to make their cpu's more consistent so the i7's can run at least 16GB of what the board is rated. As it stands you're more or less taking a chance with (the more expensive) high speed ram. I know it's based on the silicon lottery but we the people should feel more secure about purchasing high speed ram. Sure we can bin cpu's but that gets expensive fast and in the end you can only use one cpu.

So what would Intel need, new more accurate machines that makes the cpu's ? It's not like they're starving for money and I feel it could be done.

Intel rates coffee lake for up to 64GB at 2666MHz, that covers their butt pretty good. But when the motherboard supports up to 4133MHz the cpu shouldn't be the limiting factor.

Intel does not validate speeds beyond stock. That's up to the board vendor. This isn't going to change on either side no matter how much we debate ideals. So, for the masses, the safe rule of thumb is always to remove at least two ratios from the board-validated speed.